A usual heat exchanger includes a first flat tube of a flat shape having a plurality of through holes in which a low temperature fluid flows, a second flat tube of a flat shape having a plurality of through holes in which a high temperature fluid flows, a first header connected to both ends of the first flat tube and a second header connected to both ends of the second flat tube. The first flat tube and the second flat tube are laminated under a state that the flat surfaces of the tubes respectively come into contact with each other so that the longitudinal directions (the flowing directions of the fluids) of the first flat tube and the second flat tube are parallel to each other. Thus, a high efficiency of a heat exchanging performance is obtained (for instance, see Patent Document 1)    Patent Document 1: JP-A-2002-340485 (Pages 4 to 5, FIG. 1)